Surface sensing device with optical sensor

ABSTRACT

A surface sensing device for use in position determining apparatus has an elongate stylus with a tip for scanning the surface of a workpiece to be measured. Lateral displacements of the stylus tip are detected by a light beam which passes along the stylus from a light source to a retroreflector. This reflects the beam back via a beamsplitter to a position sensitive detector. The stylus is mounted for longitudinal displacement on a carriage. The longitudinal displacement is measured by another light beam projected by the beamsplitter onto a second position sensitive detector.

This is a Continuation of application Ser. No. 11/918,524 filed Oct. 15,2007, which is a National Phase of International Patent Application No.PCT/GB2006/001534 filed Apr. 26, 2006, which claims priority of BritishPatent Application No. 0508388.6 filed Apr. 26, 2005. The disclosure ofthe prior applications is hereby incorporated by reference herein in itsentirety.

TECHNICAL FIELD

The present invention relates to a surface sensing device for use inposition determining apparatus such as, for example, a coordinatemeasuring machine (CMM), a scanning machine, a machine tool or ameasuring robot.

RELATED ART

Such machines are used for measuring workpieces, and typically comprisean arm or other member which is movable in three directions X, Y and Zrelative to a table on which the workpiece is supported. Movements ofthe movable arm or other member in each of the X, Y and Z directions aremeasured by transducers on the machine, so that the position of themovable member relative to a datum position can be determined.

The surface sensing device with which the invention is particularlyconcerned is an analogue or measuring probe, having an elongate styluswith a workpiece-contacting tip. In use, it may be mounted on anarticulating head for use in a high speed scanning operation, such asdescribed in our U.S. Pat. No. 5,040,306. The head is mounted on themovable member of the machine, and has motors or actuators capable oforienting the axis of the probe stylus about two orthogonal axes ofrotation. Transducers associated with these rotatable axes determine thedirection of orientation.

During a scanning operation, the machine and/or the head cause thestylus tip to move over the surface of the workpiece, in accordance withinstructions from the machine controller, to gather data about theprofile of the workpiece surface. From the signals provided by thetransducers of the machine and the head, and from a knowledge of thedimensions of the probe stylus, the positions of points on the surfacebeing scanned can be estimated. However, this would only have therequired accuracy if the stylus were made sufficiently rigid, which isimpractical.

Our U.S. Pat. No. 6,633,051 (corresponding to International PatentApplication No. WO 00/60310) shows such a probe. It includes arelatively flexible, hollow stylus, which bends laterally under theforces of contact between the stylus tip and the workpiece surface, andunder inertial forces while accelerating. An optical system is providedwhich measures the lateral displacement of the stylus tip caused by suchbending. This is then combined with the measurements from thetransducers of the machine and the head.

The optical system comprises a light beam which passes along the hollowstylus. The beam is then reflected by an optical component at or nearthe tip, to pass back along the stylus. Lateral displacement of thestylus tip causes a lateral or tilting displacement of the returnedbeam, which is measured by a position sensitive detector.

The probe shown in U.S. Pat. No. 6,633,051 is capable of measuring onlylateral displacements of the stylus tip.

SUMMARY

The present invention provides a surface sensing device for use inposition determining apparatus, comprising:

-   -   an elongate stylus having a workpiece-sensing stylus tip;    -   an optical element at or near or connected to the tip so as to        be subjected to lateral displacements of the tip;    -   an optical transducer system which projects a light beam between        the optical element and a detector, thereby to measure said        lateral displacements of the tip;    -   wherein the stylus is mounted in the surface sensing device for        longitudinal displacement; and    -   means are provided for measuring said longitudinal displacement.

In some preferred embodiments, the means for measuring the longitudinaldisplacement of the stylus is optical.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of an articulating head with asurface sensing device;

FIGS. 2A and 2B are a cross-section of a first embodiment of surfacesensing device;

FIG. 3 is a cross-section of a second embodiment of surface sensingdevice; and

FIGS. 4,5,6 and 7 show alternative stylus assemblies for such devices.

DETAILED DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the invention will now be described by way ofexample, with reference to the accompanying drawings.

Referring now to FIG. 1 there is shown an articulating probe head. Thehead comprises a first housing part 10 adapted for attachment to aposition determining apparatus (not shown), and which contains a motoror actuator 12 to provide rotation of a shaft 14 about a first axis Z.Attached to the shaft 14 is a second housing part 16 which contains asecond motor or actuator 18 to provide rotation of a second shaft 20about a second, orthogonal axis X. Attached to the second shaft 20 forrotation therewith is a housing 24 which includes a support for asurface sensing device comprising a probe 22 and a stylus assembly 26.Angular transducers are associated with the axes X and Z, to measure therotary motion and provide feedback to a controller.

FIGS. 2A and 2B show the probe 22 and stylus assembly 26. The probe 22is exchangeably attached to the housing or support 24 of the head via aknown kinematic mount 28, into which it is attracted magnetically inknown manner, so as to be easily removable and exchangeable. Likewise,the stylus assembly 26 is exchangeably attached to the probe 22 via akinematic mount 30, into which it is similarly attracted by magnets. Aswell known, the kinematic mounts ensure that the probe and stylus arepositioned in a precisely repeatable manner, enabling precise,repeatable measurements. The probe and stylus can be exchanged foralternative probes or styli, by automatic probe/stylus changingapparatus as described in U.S. Pat. No. 5,327,657 and European PatentNo. EP 566719 (both of which are incorporated herein by reference).

The probe 22 includes a carriage 32 which is movable vertically (in theorientation seen in the drawings), i.e. in the longitudinal direction ofthe stylus. The stylus 26 is attached to this carriage 32 via thekinematic mount 30. The carriage 32 is mounted to the relatively fixedstructure 33 of the probe 22 via two horizontally extending planar leafsprings or diaphragms 34, which permit the vertical movement butconstrain lateral movements in X and Y directions.

The stylus assembly 26 includes an elongate hollow tubular stylus 36,made of carbon fibre. This is slightly flexible (resiliently) in thelateral X, Y directions and its stiffness and weight are designed togive good dynamic performance under high speed scanning conditions. Ithas a stylus tip 38 which contacts the workpiece during such scanning.Rather than being hollow, the stylus 36 could if desired be made of asolid transparent material such as a suitable glass.

During scanning movements, the stylus tip 38 will undergo verticalmovements, permitted by the planar springs 34. It will also undergolateral X, Y movements permitted by bending of the stylus 36. The probe22 contains transducers to measure these movements, which will now bedescribed.

The probe 22 includes a laser diode or other light source 40. Thisproduces a beam of light which is collimated or brought to a focus by alens 42. The laser diode is mounted on an adjustable clamp for alignmentpurposes, such that in conjunction with the lens 42 the light beampasses axially along the hollow stylus 36. Both the laser diode 40 andthe lens 42 are provided on the fixed structure 33 of the probe 22.

A beamsplitter 44 is also provided on the fixed structure, to receivethe light beam emitted by the lens 42. It passes 50% of the light downthe stylus 36. Near the stylus tip 38, a lens 46 and a minor surface 50(provided on a glass cylinder 48) act as a retroreflector, to return thelight beam back along the length of the stylus 36.50% of the returnedbeam is reflected through 90° by the beamsplitter 44, onto atwo-dimensional position sensitive detector 52 (which is located on thefixed structure 33 of the probe). This arrangement is similar to thatdescribed in U.S. Pat. No. 6,633,051. Any of the other arrangementsdescribed in U.S. Pat. No. 6,633,051 could be used instead, and thatspecification is hereby incorporated by reference.

When the stylus tip 38 is deflected laterally in X or Y directions, theretroreflector (optical element) 46,50 causes the returned beam to belaterally displaced by a corresponding amount in the correspondingdirections X, Y. This is detected by the position sensitive detector 52.In the embodiment of FIGS. 2A and 2B, the optical transducer systemincludes the laser diode or other light source 40, lens 42, beamsplitter44 and detector 52. Alternatively, with some of the arrangements shownin U.S. Pat. No. 6,633,051, the lateral displacement of the stylus tip38 could cause a tilting displacement of the returned beam, which againis measured in the X and Y directions by the position sensitive detector52.

The distance and the focus of the mirror/lens combination 46,50 may beadjustable. This enables adjustment of the “gain” of the probe, i.e.matching the amount of displacement of the returned beam at the detector52 depending on the length of the stylus and the resulting amount bywhich the stylus tip 38 is displaceable laterally.

The other 50% of the beam emitted by the laser diode 40 and lens 42 isreflected laterally towards a second position sensitive detector 54.This is also mounted on the fixed structure 33 of the probe. However,between the beamsplitter 44 and the position sensitive detector 54, thebeam passes through a lens 56 which is mounted on the vertically-movablecarriage 32. Thus, the vertical position of the beam on the detector 54is deflected, depending upon the vertical position of the carriage 32.Since the stylus assembly 26 is rigid in the longitudinal direction, theoutput of the detector 54 is a direct measure of the longitudinalposition (Z) of the stylus tip 38. The response of the detector 54 to agiven amount of vertical movement may be amplified by an appropriatechoice of lens 56.

The position sensitive detector 54 could merely be a one-dimensionaldetector. However, it is more convenient to use a two-dimensionaldetector, the same as the detector 52, simply ignoring the output forthe other dimension.

The outputs of the detectors 52,54 therefore give a direct indication ofthe three-dimensional position of the stylus tip 38, relative to thehousing 24 of the articulating head. This can be combined in a knownmanner with the outputs of transducers in the head and of the machine,in order to determine the tip position during a scanning operation.

Other detectors can be used as the position sensitive detectors, e.g. aCCD or other camera chip, or a quad cell.

FIG. 3 shows a cross-section through an alternative embodiment. A probe60 is mounted on an articulating head 62, and has an exchangeable,kinematically mounted stylus assembly 64. A laser diode or other lightsource 66, lens 68 and beamsplitter 70 are mounted on the relativelyfixed structure of the probe, and produce light beams in a similarmanner to FIG. 2A. In the embodiment of FIG. 3, the optical transducersystem includes the laser diode or light source 66, the lens 68, thebeamsplitter 70 and a position sensitive sensor 76. The stylus assembly64 is mounted on a vertically-movable carriage 72 via two parallelplanar leaf springs or diaphragms 63, also as in FIG. 2A.

One of the light beams passes down a hollow tubular stylus 74 of thestylus assembly 64, and is retroreflected back up the stylus anddirected by the beamsplitter 70 to an X, Y position sensitive detector76. Again, this is similar to FIGS. 2A and 2B, except that this time theretroreflector (optical element) is formed by a GRIN lens 78 with amirrored back surface 80. This arrangement measures the X, Y deflectionsof the stylus tip 82, as previously. In this embodiment, contrary to theprevious embodiment, the vertical (Z) movement of the carriage 72 (andthus of the stylus tip 82) is measured by a position sensitive detector84 which is mounted on the vertically-movable carriage 72.

Again therefore, the outputs of the detectors 76,84 provide a directmeasurement of the movement of the stylus tip 82 in X, Y and Zdirections, relative to the head 62.

Other arrangements can be envisaged in order to measure the stylus tipmovements. For example, the movement in the Z direction could bemeasured by strain gauges mounted on the planar springs which permit thevertical movement of the carriage 32 (FIG. 2A) or 72 (FIG. 3). Oranother transducer can be provided to measure the vertical movementbetween the carriage and the fixed structure. Or the distance along thelight beam passing through the stylus can be measuredinterferometrically, or by the time of flight of the light beam.

Other arrangements of the beamsplitter 42 or 70 and the detectors 52,54or 76,84 can be envisaged. For example, in FIG. 3, the beamsplitter 70and detector 76 could both be mounted on the movable carriage 72, inorder to measure the X, Y movements. The detector 84 would then bemounted on the relatively fixed structure of the probe, so that thevertical movements of the beamsplitter 70 cause the light beam to movevertically on the detector 84 and thus measure the vertical movements ofthe stylus tip 82.

In another embodiment, instead of providing a retroreflector, the laserdiode or other light source 40 may be located at the bottom end of thestylus 36 with a lens 120, as shown in FIG. 7. Or it may actually be inthe stylus tip 38. It then directs a light beam up the stylus to thebeamsplitter. Electrical connections to the laser diode may be providedintegrally in the side wall of the stylus.

Alternatively in FIG. 7, the detector 52 may be located at the bottomend of the stylus or in the stylus tip, optionally without the lens 120,to receive light from a light source positioned within the probe body asin FIG. 2A. This has the advantage that the arrangement detects onlylateral movement of the stylus tip, as required, and is insensitive totilting of the stylus.

The above embodiments have included a stylus tip which contacts theworkpiece during a scanning operation. However, the invention is alsouseful with a non-contacting tip, which senses the workpiece surfaceusing a non-contact transducer, e.g. capacitively, inductively oroptically. The detectors 52,54 or 76,84 then measure stylus deflectionscaused e.g. by inertial forces during the accelerations of the scanningmovement or drooping under gravity.

In the above embodiments, FIGS. 2B and 3 show a stylus extension 86which mounts the stylus tip 38,82 to the end of the stylus 36,74. Thismay be longer than shown where longer styli are required for aparticular measurement task. The stylus extension may be made stifferthan the stylus 36,74 so that most bending occurs in the stylus 36,74.Or it may have a similar stiffness. In either case, the bending measuredin the stylus 36,74 is proportional to the bending of the total assemblyof the stylus and extension.

FIG. 4 shows another embodiment of the stylus assembly, which mayreplace the stylus assemblies 26,64 in FIGS. 2B and 3. A hollow,relatively flexible stylus 88 has a stylus tip 90, as previously. Withinthe stylus 88 is a relatively stiff rod 92. At one end 94 this isattached at or near the stylus tip 90. The other end 96 is free.

At the free end 96 of the rod is located a retroreflector 98 of anysuitable design. This reflects the light beam from and to thebeamsplitter 44,70 as in previous embodiments. Since the rod 92 isrelatively stiff, the Movement of its free end 96 follows the movementof the tip 90 as the stylus 88 bends.

Alternatively, as shown in FIG. 5, there may be an X, Y transducercomprising a first element 100 on the free end 96 of the rod 92, and asecond element 102 on the fixed structure of the probe. This X, Ytransducer may be capacitive, inductive (e.g. eddy current) or anoptical encoder.

FIG. 6 shows an alternative stylus assembly. It has a stylus 104 whichis mounted on a planar spring or diaphragm 106. Thus, effective bendingof the stylus assembly is provided by the bending of the planar spring106 (as shown in broken lines) instead of relying on the bending of thestylus 104 itself. Another alternative would be to weaken the tubularstylus 104 by local perforations near its mounting point, in order toallow it to bend while retaining stiffness in the Z direction.

The upper end of the stylus 104 is provided with a suitableretroreflector e.g. comprising a lens 108 and mirror 110. This returns alight beam from and to the beamsplitter 44,70 as in the previousembodiments. If the lens 108 is omitted, the beam can still be returnedin the same way, but will be tiltingly displaced instead of laterallydisplaced.

1. A surface sensing device for use in position determining apparatus,comprising: an elongate stylus having a workpiece-sensing stylus tip,wherein the elongate stylus has a relatively flexible portion permittingbending and a stylus extension, the stylus extension is stiffer than therelatively flexible portion, the stylus extension is closer to thestylus tip than the relatively flexible portion is to the stylus tip,and the stylus tip is subjected to lateral displacements caused by thebending; and a transducer system which measures lateral displacements ofthe stylus tip, including those caused by the bending.
 2. A surfacesensing device according to claim 1, wherein the transducer system isoptical.
 3. A surface sensing device according to claim 2, wherein thetransducer system includes an optical element at or near or connected tothe stylus tip so as to be subjected to the lateral displacements of thestylus tip, and a detector, and wherein the transducer system projects alight beam between the optical element and the detector.
 4. A surfacesensing device according to claim 1, comprising: a mounting for theelongate stylus in the surface sensing device which enables longitudinaldisplacement of the elongate stylus; and a displacement measurementtransducer arranged to measure the longitudinal displacement of theelongate stylus.
 5. A surface sensing device according to claim 4,comprising a probe, wherein the probe comprises a longitudinally movablecarriage, to which the elongate stylus is exchangeably attached, andwherein the displacement measurement transducer is arranged to measurelongitudinal displacement of the carriage.
 6. A surface sensing deviceaccording to claim 1, comprising a probe, wherein the elongate stylus isexchangeably attached to the probe.
 7. A surface sensing deviceaccording to claim 6, wherein the exchangeable elongate stylus adjuststhe gain of the transducer system which measures the lateraldisplacements of the stylus tip.
 8. A surface sensing device for use inposition determining apparatus, comprising: an elongate stylus having aworkpiece-sensing stylus tip, wherein the elongate stylus has arelatively weakened portion permitting bending and a stylus extension,the stylus extension is stiffer than the relatively weakened portion,the stylus extension is closer to the stylus tip than the relativelyweakened portion is to the stylus tip, and the stylus tip is subjectedto lateral displacements caused by the bending; and a transducer systemwhich measures lateral displacements of the stylus tip, including thosecaused by the bending.
 9. A surface sensing device according to claim 8,wherein the relatively weakened portion comprises local perforations ofthe elongate stylus.
 10. A surface sensing device according to claim 8,wherein the transducer system is optical.
 11. A surface sensing deviceaccording to claim 10, wherein the transducer system includes an opticalelement connected to the stylus tip so as to be subjected to the lateraldisplacements of the stylus tip, and a detector, and wherein thetransducer system projects a light beam between the optical element andthe detector.
 12. A surface sensing device according to claim 8,comprising: a mounting for the elongate stylus in the surface sensingdevice which enables longitudinal displacement of the elongate stylus;and a displacement measurement transducer arranged to measure thelongitudinal displacement of the elongate stylus.
 13. A surface sensingdevice according to claim 12, comprising a probe, wherein the probecomprises a longitudinally movable carriage, to which the elongatestylus is exchangeably attached, and wherein the displacementmeasurement transducer is arranged to measure longitudinal displacementof the carriage.
 14. A surface sensing device according to claim 8,comprising a probe, wherein the elongate stylus is exchangeably attachedto the probe.
 15. A surface sensing device according to claim 14,wherein the exchangeable elongate stylus adjusts the gain of thetransducer system which measures the lateral displacements of the stylustip.